There are numerous metal cutting applications requiring that a 90° shoulder be cut in a work piece. To make a 90° shoulder, a cutting insert seated in a tool preferably employs an operative primary cutting edge in conjunction with a wiper (sometimes referred to as a ‘secondary’ cutting edge) to which it is connected via a corner. Such a cutting insert needs the following clearances with respect to the tool: a primary cutting edge relief angle, a wiper relief angle, and relief between any non-operative cutting edges at the work piece.
U.S. Pat. No. 6,336,776 discloses a fully indexable negative geometry polygonal insert for machining a square (or 90°) corner, in a work piece. The disclosed insert uses an operative primary cutting edge extending in a first direction from each corner of the polygon downwardly from an associated major polygonal surface of the insert along with a wiper lying substantially in the plane of the associated major surface and extending in a second direction from the insert corner. Instead of being directly connected to the insert corner, the wiper is spaced apart from the insert corner by a transitional cutting edge.
The geometry of the primary cutting edge enables mounting of the insert to a milling cutter with negative axial rake, such that the primary cutting edge is substantially parallel to an axis of rotation of the cutting tool. The wiper laying on the major surface does not have a positive rake face associated with it. Thus, when the insert is seated to provide the primary cutting edge with negative axial rake and negative radial rake, an acute angle is formed between the wiper and the work piece, resulting in a double negative cutting geometry, which may adversely impact surface finish.
Also, when the insert is positioned on the tool the primary cutting edge is parallel to the axis of rotation, i.e., the axial angle is zero. This exerts a high load on cutting because the entire cutting edge experiences the material all at once, and there is no gradual entry of the cutting edge in the material.